The present invention relates to a method for controlling the detoxification of aqueous solutions, in particular waste waters, containing cyanides and/or unstable cyano complexes. In particular the present invention relates to a method for treating waste waters containing zinc cyano complexes, wherein in a first stage free cyanide and that released from cyano complexes is converted into glycolnitrile by the addition of formaldehyde or a compound capable of releasing formaldehyde under the process conditions. In a second stage the glycolnitrile formed in the first stage is decomposed by perhydrolysis by the addition of a source for hydrogen peroxide.
The detoxification of cyanidic waste waters, such as are obtained in various branches of industry, for example in the mining industry, in metal hardening plants and electroplating works, with the use of per-oxygen compounds, such as in particular hydrogen peroxide and persulphates, has been known for a long time. In order to reduce the reaction time for the detoxification, the additional use of heavy-metal catalysts is often necessary--see French patent 1 564 915. A disadvantage with the additional use of heavy-metal catalysts is that in many cases they catalyze the decomposition of the hydrogen peroxide before completion of the cyanide detoxification, so that a sufficient degree of detoxification is not achieved. A further disadvantage of this known method with the use of heavy-metal catalysts resides in the fact that the recovery in sufficiently pure form of useful materials contained in the waste water to be treated, for instance the recovery of zinc from waste waters from cyanidic galvanizing, is prevented or rendered unduly complicated.
According to another known method hydrogen peroxide is used for the detoxification of cyanidic solutions in combination with formaldehyde; see U.S. Pat. No. 3,617,582. The reaction takes place in two stages, wherein in the first stage formaldehyde cyanohydrin, which is also known as glycolnitrile, is formed. In the second stage perhydrolysis with the use of hydrogen peroxide is involved, wherein glycolnitrile is decomposed to glycolic acid amide and glycolic acid. In the method according to the above-mentioned U.S. Pat. No. 3,617,582 the detoxification takes place at a pH value in the range from 9 to 12.5. Formaldehyde can be used in the form of an aqueous solution or a compound releasing formaldehyde, such as for example paraformaldehyde. Instead of hydrogen peroxide, compounds releasing hydrogen peroxide, such as perborates or percarbonates, can also be used for the perhydrolysis. Such sources of hydrogen peroxide are well known in the art. Formaldehyde and hydrogen peroxide are added to the solution to be detoxified in particular molar ratios, calculated on the previously determined content of cyanide. A suggestion for controlling the detoxification of solutions, the cyanide content of which is unknown, is not given in this prior art document; that is, in the prior known method the cyanide content always had to be determined prior to the actual detoxification. In the case of detoxification methods in particular, however, there is an interest at a practical level in easily controllable methods which make a cyanide determination prior to the detoxification superfluous.
A method is known from U.S. Pat. No. 4,731,232 for the purifying of gas wash waters containing cyanides, which is based on the treatment of the wash water with formaldehyde and a subsequent oxidation step with the use of hydrogen peroxide. In this method the charging of formaldehyde takes place as a function of the cyanide-specific redox potential, which is measured in a side flow diverted from the main flow with the use of a noble metal electrode and a reference electrode. Because of the pH-dependence of the redox potential the pH value of the side flow is adjusted to a value between 7 and 10 and kept constant. In addition silver ions are introduced into the side flow. The regulation of the addition of hydrogen peroxide takes place once again as a function of the redox potential measured. It is a disadvantage of the method described that limits are set to a control of the addition of the required chemicals by means of the measured redox potentials, because always only one mixed potential is measured. In the case of solutions with fluctuating compositions, such as are obtained in particular in electroplating plants, the above-mentioned control technique is therefore often not sufficient, because either under-charging or over-charging of formaldehyde can occur. This is accompanied in some cases with an insufficient degree of detoxification and/or an excessive consumption of chemicals.